The evaluation of the functional activity of the thyroid requires measurement of the ability of the thyroid gland to trap, organify, store, and secrete iodine. Recently a new technique (XRF) has been developed to determine the steady state intrathyroidal iodine pool size. This parameter reflects the past history as well as the present functional characteristics of the gland and should provide useful clinical and research information on thyroid patho-physiology. The use of x-ray fluorescence (XRF) scanning permits one to obtain a map and measurement of the iodine content of the thyroid in-vivo. A very small radiation dose is delivered to the thyroid, and a negligible dose is absorbed outside the irradiated field. We plan to evaluate the role of XRF scanning in the diagnosis of thyroid disorders. In particular, a ratio value comparing the iodine content in a nodule to that in a corresponding area of the contralateral lobe appears to be an excellent predictor of malignancy and detailed studies are now under way to verify this predictor. The results of the fluorescence scan will be compared to conventional radio-isotope studies and ultrasonic A-scans to determine what additional information is provided by the fluorescence scan in each of several patient groups. We hope to demonstrate the benefits to be derived from this technique, so that other clinics may be encouraged to use this technique for thyroid evaluations.